Mechanical unloaders and variable frequency drives (VFDs) are alternative means of reducing compressor capacity to correspond to loads below the maximum compressor capacity. Reducing compressor capacity when load is below the maximum compressor capacity allows the compressor to save some energy. VFDs allow compressor capacity to be reduced by reducing compressor speed when capacity exceeds load. Mechanical unloaders reduce compressor capacity by affecting the volume ratio of the compressor. VFDs and mechanical unloaders are typically applied separately, using only one or the other in a compressor design, or when both are features of a compressor, they are used sequentially, reducing compressor speed to a minimum level via the VFD before mechanically unloading the compressor.
For example, when a compressor is to be operated at 90% capacity, a VFD may reduce its speed from 60 Hz to 54 Hz. Mechanical unloaders may only be used once a minimum speed threshold for compressor operations, for example 30 Hz, has been met through the VFD, and only then will mechanical unloaders be used to adjust the volume ratio of the compressor to further reduce capacity from 50% of capacity on down to the minimum capacity of the compressor.